docs: def appndd(item, cell):
      """
      Append an item to a double precision cell.
  
      http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/appndd_c.html
  
      :param item: The item to append.
      :type item: Union[float,Iterable[float]]
      :param cell: The cell to append to.
      :type cell: spiceypy.utils.support_types.SpiceCell
      """
      assert isinstance(cell, stypes.SpiceCell)
      if hasattr(item, "__iter__"):
          for d in item:
              libspice.appndd_c(ctypes.c_double(d), cell)
      else:
          item = ctypes.c_double(item)
          libspice.appndd_c(item, cell)